This is a resubmission of a proposal requesting funds for the purchase of a Philips CM 10 electron microscope (to replace an obsolete Siemens 1A EM) to be used by investigators of the Departments of Medical and Molecular Parasitology, Microbiology, and the Division of Immunology (Department of Pathology), of New York University. The close scientific collaboration among several investigators of these Departments makes it highly desirable to have one shared, efficient electron microscope facility. Ongoing work in these departments, will benefit greatly from the acquisition of the requested microscope. Our current and proposed projects encompass research on several ultrastructural aspects of microbial-host cell relationships and defense mechanisms. For this purpose we will: a) determine the subcellular localization of plasmodial antigens by immuno-E.M and perform a molecular analysis of Plasmodium vivax membrane antigens; b) determine the characteristics of sporozoite motility and invasiveness, investigate by immuno-EM. sporozoite-host cell interactions and define anti-plasmodial mechanisms of T cells; c)define the ultrastructural characteristics of the proliferative stages of Pneumocystis carinii; d) clarify the mechanisms of attachment and penetration of Trypanosoma cruzi into host cells, perform ultrastructural studies on antigen processing and on mechanisms of inhibition of complement mediated lysis.; e) perform studies on the microbicidal activity of leucocytes and bacterial phospholipid degradation; f) perform studies on the regulation of sporulation in Bacilli; g) characterize the ENV proteins of the AIDS virus and investigate the viral etiology and pathogenesis of Kaposi sarcoma and h) investigate the viral and cellular gene expression of growth regulation.